1. Field of the Invention
This invention relates to a brake system for vehicles, and more particularly to improvements in a booster unit actuating a master cylinder in such a brake system.
2. Description of the Prior Art
FIG. 1 shows schematically the structure of a prior art booster unit incorporated in a brake system for vehicles. As shown in FIG. 1, the prior art booster unit 01 includes a piston 02 operatively connected to a master cylinder (not shown), a regulator 03 disposed slidably in the piston 02 and connected at its left-hand end to a brake pedal (not shown) for movement by pedal depression and a return spring 04 interposed between the regulator 03 and the piston 02.
In response to the depression of the brake pedal for the purpose of braking, the regulator 03 is urged rightward in FIG. 1 against the force of the return spring 04, and a valve mechanism 05 is actuated to establish communication between a front chamber 06 and a pressure chamber 07, with the result that the piston 02 is urged rightward in FIG. 1 by the internal pressure of the pressure chamber 07. When the rightward stroke of the piston 02 becomes equal to the leftward stroke of the regulator 03, the valve mechanism 05 interrupts the communication between the front chamber 06 and the pressure chamber 07, and the force urging the piston 02 rightward is balanced by the reaction force of the master cylinder to stop the piston 02 in the balanced position. Then, when the brake pedal is depressed again, the booster unit 01 operates again in a manner as described above.
Therefore, in the brake system including the prior art booster unit 01, the reaction force (the brake pedal depression force) imparted through the regulator 03 is counteracted by the force of the return spring 04 compressed due to the relative displacement of the regulator 03 and piston 02, and such a force of the spring 04 is almost constant irrespective of the displacement of the brake pedal. The brake system including the prior art booster unit 01 has not thus been satisfactory in the feeling of brake pedal actuation when compared with a conventional brake system in which the brake pedal depression force increases with the increase in the brake pedal displacement.